Effects of temperature, NaCl, and methicillin on penicillin-binding proteins, growth, peptidoglycan synthesis, and autolysis in methicillin-resistant Staphylococcus aureus

Abstract

Methicillin-resistant Staphylococcus aureus strains produce a fifth penicillin-binding protein (PBP), PBP 2', with low affinity for beta-lactam antibiotics that is believed to represent a beta-lactam-insensitive peptidoglycan transpeptidase. In an effort to evaluate the adequacy of PBP 2' as an explanation of methicillin resistance, PBP 2' production and the responses of growth and peptidoglycan synthesis to methicillin under different environmental conditions have been compared. In the heterogeneous methicillin-resistant strain DU4916-K7, less PBP 2' was produced at 40 degrees C than at 30 degrees C, but inclusion of 5% (wt/vol) NaCl in the medium at 40 degrees C boosted PBP 2' production and allowed growth of the organism in the presence of 10 micrograms of methicillin per ml. When exponential-phase cultures were challenged with methicillin, growth and peptidoglycan synthesis were much more resistant at 30 degrees C than at 40 degrees C. Inclusion of NaCl in medium rendered growth and peptidoglycan synthesis more methicillin resistant at 40 degrees C. Hence, there was a good correlation between PBP 2' production and methicillin-resistant peptidoglycan synthesis under these conditions. However, PBP 2' production was increased by NaCl at 30 degrees C without markedly affecting the susceptibilities of growth and peptidoglycan synthesis to methicillin. Pregrowth of cells with methicillin, which was expected to boost PBP 2' production, seemed to increase the susceptibilities of growth and peptidoglycan synthesis to methicillin. Patterns of growth and peptidoglycan synthesis susceptibilities to methicillin which were similar to those described above were found in chloramphenicol-inhibited cultures, in which presumably no induction of PBP 2' could occur during the methicillin challenge period. Complex effects were noted in the combination of subinhibitory methicillin and NaCl. Growth of cells in the presence of NaCl stimulated their autolytic activity, which was further increased by growth with subinhibitory methicillin in addition to NaCl. It appears that NaCl enhances methicillin resistance by stimulating PBP 2' production and providing osmotic support but opposes it by stimulating autolytic activity which is exacerbated by the very low cross-linking of peptidoglycan in methicillin-resistant strains grown in the presence of methicillin.